I suggest you might try getting an exact measurement of the barrel's bore and groove diameter, either by slugging the bore or making a cast. In my experience, factory rifles in 6.5X54 M/S tend to run .268" in the bore, rather than the more usual 6.5mm diameter of .264". Hornady makes .268" bullets for use in the Mannlicher Carcano rifle, and you may be able to locate some in the UK.